How to calculate BTU per passenger mile from miles per gallon

A value of 125,000 is generally used as the heat content of one gallon of gasoline. (138,700 for Diesel)

If one gallon takes you 10 miles, then each mile required 1/10 of a gallon.

Expressed as an equation (Applying rules of fractions to the units):

Next you can adjust for more than one passenger. (The driver of a car is a passenger, the bus driver is not):

For 32 MPG:
125,000 / 32=3,906 per vehicle mile
3,906 / 1.2 passengers = 3,255 BTU per passenger-mile

For 60 MPG:

125,000 / 60 = 2,083 BTU per vehicle mile
2,083 / 2 passengers = 1,042 BTU per passenger-mile

Analysing Trimet's claimed mileage for cars and SUVs:

14 mpg:
125,000 divided by 14  mpg =  8,928 BTU per vehicle-mile.  8,928 / 8,150 = 1.10 passengers

17 mpg:
125,000 divided by 17 mpg =  7,352BTU per vehicle-mile.  7,352 / 6,712  = 1.19 passengers